Today I Learned

[mfb]

It also works for hydrogen -> protonium

Are you sure it doesn't work for other elements? I would expect the lifetime to go down, so experimental observation is more challenging, and also the analysis gets more complicated. Maybe no one did it so far because it is not very attractive.

 

[collinsmark]

According to the antiprotonic helium wiki,

"The antiproton can thus orbit the nucleus for tens of microseconds, before finally falling to its surface and annihilating. This long lifetime is seen only in the case of antiprotons mixed in helium; in all other substances, antiprotons annihilate a million times faster, in less than a picosecond."​

 

[mfb]

Well, that is wrong, removed it. Protonium for 1.1µs (page 9) - and higher l would lead to longer lifetimes.

Edit: typo

 

[Jonathan Scott]

Well, that is wrong, removed it. Positronium for 1.1µs (page 9) - and higher l would lead to longer lifetimes.

You mean Protonium, I presume (combination of antiproton and proton), although that isn't really like an element, and it also appears that Protonium is only stable in isolation, not as a gas like antiprotonic helium. Also, it is not created by just mixing antiprotons into hydrogen; it requires a magnetic trap to mix antiprotons and protons.

I don't see any problem with the original specific statement: "This long lifetime is seen only in the case of antiprotons mixed in helium; in all other substances, antiprotons annihilate a million times faster, in less than a picosecond."

 

[fresh_42]

I don't see any problem with the original specific statement

That's what I thought. But Positronium is interesting, too. And with 142 ns is Orthopositronium almost a Methuselah.

 

[mfb]

I meant protonium of course.

antiprotons annihilate a million times faster, in less than a picosecond.

Well, they don't do that in general, as the reference shows. You can always find conditions where things live shorter, but that is not the point. The statement claimed that it cannot live longer than a picosecond, which is wrong by at least 6 orders of magnitude.

 

[Jonathan Scott]

I meant protonium of course. Well, they don't do that in general, as the reference shows. You can always find conditions where things live shorter, but that is not the point. The statement claimed that it cannot live longer than a picosecond, which is wrong by at least 6 orders of magnitude.

The relevant sentence specifically applied to antiprotons being mixed with "substances" of normal matter.
The second reference starts with the following:

An antiproton introduced into ordinary matter will usually annihilate with a nucleus within picoseconds. Only one exception is so far known to this general rule: About 3% of antiprotons stopped in helium can survive up to some tens of microseconds [1–7], an effect that has been ascribed to the formation of long-lived $\bar p {\rm He}^{+} \, \equiv \, \bar p {-e}^{-} {\rm He} ^{2+}$ exotic atoms [8–16].

 

[mfb]

Usually.

A "substance" can have a low pressure, making annihilations less frequent.

 

[Jonathan Scott]

Usually.

A "substance" can have a low pressure, making annihilations less frequent.

In the reference, the only exception to "usually" is the helium case. Are you saying that you have better information than the reference?

If you're saying that the lifetime can be extended by having the "substance" at such a low pressure that the antiproton has a long free path before it encounters anything, I don't think it is sensible terminology to refer to that as mixing it with ordinary matter until it actually encounters something!

 

[mfb]

See my previous reference: Protonium can be long-living, and it is "antiprotons introduced into ordinary matter" as hydrogen atoms are ordinary matter, at any pressure.

 

[Jonathan Scott]

See my previous reference: Protonium can be long-living, and it is "antiprotons introduced into ordinary matter" as hydrogen atoms are ordinary matter, at any pressure.

The Wikipedia page for protonium (which was the source of my previous information about them) says that protonium is either produced in "violent particle collisions" or by "putting antiprotons and protons into the same magnetic cage". I don't think either can be described as mixing antiprotons with ordinary matter.

I still think that given the referenced information, the original statement about mixing antiprotons with substances appears to be helpful and accurate.

 

[fresh_42]

Mais, Messieurs, contenance!

 

[mfb]

"Putting two things together at the same place" is as much "mixing" as it can get.

 

[Jonathan Scott]

"Putting two things together at the same place" is as much "mixing" as it can get.

True, but protons are not "ordinary matter" unless they have matching electrons to make hydrogen.

 

[fresh_42]

TIL that one of my favorite cookies, which I always thought of being a local specialty, is indeed one. Only that local does not mean local to me but Dutch and Belgian instead. And I learned that it had been something for the upper class due to the fact, that spices had been rather expensive in older times. And the answer to the most exciting question: "Where does the name come from?" has been a disappointing: "No one knows for certain."

 

[DennisN]

Today I learned

 

[beamie564]

Today I learned how a guitar tuner works. I've started learning how to play one. I've also started practising playing the keyboard.

 

[jtbell]

Today I learned that squirrels eat flowers. (not just nuts or acorns!)

Our deck is surrounded (on its open sides) by camellias. Today I noticed a small pile of camellia petals on the deck railing. My wife told me that this morning when she looked out the window she saw a squirrel sitting on the railing, chomping on a flower. Then it dived into the adjacent camellia and emerged with another flower.

 

[BillTre]

Today I Learned that some guy (makendo) at Instructables made a black hole table with an infinity mirror set-up (ring of LEDs between a mirror and a partial mirror) at the bottom so it will optically go through the floor.

 

[BillTre]

Today I learned (while going along on a cornea "recovery"):
that the irises of blue-eyed people are less adherent to the cornea than those of brown eyed people.

 

[fresh_42]

Nice thought. I hope you'll find a lot of occasions on PF to change your mindset - the best way to learn something.

 

[Jonathan Scott]

Today I learned that I've been doing a frightening amount of unlearning over the years.

A few days ago, I remembered that I'd derived some results about expressing the "square root" of a four vector (used in a Lorentz transformation) in terms of the vector components, but I couldn't remember the details. I managed to derive the basic formula again and then remembered that it's the same thing as the half-angle formula for cosh and sinh, which I could have found online. Today, I opened an old drawer and found in it my own reference notes from 1994 summarizing all of the relevant results in lots of interesting ways, with some updates from 2007 showing for example how to derive Thomas precession from the same formula.

My brain is definitely going rusty and seizing up with lack of use.

One thing I keep meaning to do is to dig through more of my old notes trying to computerise them (which also helps refresh my memory in the process). However, I have difficulty deciding when to use Word with the Equation Editor and when to use LaTeX. I usually I find the equation editor easier for getting going quickly, but if a document grows too much I eventually either find that the equations start behaving in weird ways or that I have to spend hours trying to find out how to stop them wriggling away. Also, when Word documents get too large the editing starts to get sluggish, especially for equations, but there isn't a convenient way of maintaining a Word document as lots of separate files. With LaTeX, the main text is more boring to enter but everything seems to behave more predictably, and it's much easier to work with lots of separate small files.

 

[jedishrfu]

Today I learned that I've been doing a frightening amount of unlearning over the years.

...

With LaTeX, the main text is more boring to enter but everything seems to behave more predictably, and it's much easier to work with lots of separate small files.

Why not just scan them into a PDF and make them searchable? You'll then have something to reference and convert to latex if the need arises but you'll probably forget that you even scanned them by then. :-)

 

[fresh_42]

Today I've read the original papers of Emmy Noether with her famous theorems about symmetries and invariants.
A century is a lot of time ...

 

[Astronuc]

Today I learned that squirrels eat flowers. (not just nuts or acorns!)

Our deck is surrounded (on its open sides) by camellias. Today I noticed a small pile of camellia petals on the deck railing. My wife told me that this morning when she looked out the window she saw a squirrel sitting on the railing, chomping on a flower. Then it dived into the adjacent camellia and emerged with another flower.

One can add berries, e.g., raspberries and blueberries. I think chipmunks do the same.

 

[1oldman2]

Today I learned to differentiate between a Tomte and a Blip.

 

[rbelli1]

Today I learned to differentiate between a Tomte and a Blip.

Please elucidate.

BoB

 

[1oldman2]

Please elucidate.

BoB

They are glitch classifications used by LIGO in their detectors. Very similar shapes, you have to see them evolve over time to tell the difference.

 

[Jonathan Scott]

They are glitch classifications used by LIGO in their detectors. Very similar shapes, you have to see them evolve over time to tell the difference.

The Swedish "Tomte" is something like a garden gnome, with a tall pointed hat, so I guess (a) the glitch profile was in some sense like a pointed hat and (b) there was someone Swedish on the team!

 

[1oldman2]

The Swedish "Tomte" is something like a garden gnome, with a tall pointed hat, so I guess (a) the glitch profile was in some sense like a pointed hat and (b) there was someone Swedish on the team!

(a) would be correct.

 

[mfb]

(b) is also correct (going by recent affiliation). C. Kim, Lund observatory, Sweden. Source

 

[1oldman2]

(b) is also correct (going by recent affiliation). C. Kim, Lund observatory, Sweden. Source

That makes sense, I wondered about how Tomte was chosen as a label. there are some interesting "glitch names" used, the Koi Fish, Paired Doves and of course the ever popular "Chirp" are easy enough but the "Scattered Light" and "500hz Violin Mode Harmonic" glitch are proving to be a little more confusing.

 

[dkotschessaa]

Today I learned that

[lamé] is a type of fabric with glittery metal woven into it.

-Dave K

 

[Jonathan Scott]

Yesterday I learned that a neat and simple gravitational equation of motion which I worked out for myself in about 1986 (and assumed everyone familiar with GR would know) was considered sufficiently novel in 2013 that someone published a 15-page paper about it (and some related forms) in a peer-reviewed journal. No wonder I couldn't find references to back it up, and I got more than one infraction because my own derivation was not considered an acceptable reference.

But I'm shocked that it isn't better known. I didn't invent any new physics; I was just trying to express the equation of motion in the most Newtonian-compatible terms that I could, using coordinate time and coordinate momentum instead of proper time and coordinate velocity. All I did was shuffle standard stuff around, change some notation and spot some ways to simplify the result, until it was in a form easily understandable in terms of Newtonian concepts and comparable with the Newtonian result.

 

[jim hardy]

Point us toward said paper ? I like "Newtonian concepts" and need "easily understandable".